Automated electrophotographic copiers and printers have been known for nearly fifty years. Copiers and printers differ only from an input standpoint, copiers being adapted to receive hard copy input, whereas printers are adapted to receive an input in electronic form, e.g., from a computer terminal. Both carry out the basic electrophotographic imaging process of uniformly charging a photoconductive layer with electrostatic charge, imagewise exposing the charged layer to radiation adapted to discharge the layer, thereby leaving behind a latent charge image, and applying pigmented electroscopic particles (toner) to the charge image to render it visible. Most often, the toner image so formed is transferred to a receiver sheet whereupon the toner image is permanentized by heat and/or pressure. Optionally, for example, to extend the lifetime of the photoconductive recording element, the toner image formed on the image-recording drum is transferred to an intermediate transfer drum or the like before it is again transferred to the receiver sheet. In the case of full color copying and printing, multiple color-separated toner images (e.g., cyan, magenta, yellow and black) are produced by the above process and transferred in registration to a receiver sheet.
Since the inception of electrophotographic printers/copiers, the "holy grail" for many manufacturers has been to produce images of photographic quality, both monochromatic and full color. As will be appreciated, the quality of a full color image is determined not only by the respective qualities of each of the color-separated toner images formed on the photoconductive recording element(s), but also by the degree with which such images can be transferred from the recording element(s) and brought into perfect registration on the image receiver sheet. Such image quality of the color-separated images and the registration thereof, in turn, depend in large part upon the precision with which the various work-stations or subsystems that carry out the electrophotographic process can be physically placed relative on the surface of the recording element. Thus, various schemes have been proposed and used in the past that address this technical problem.
While focusing on image-quality and registration issues, printer manufacturers are ever mindful of lowering manufacturing and service costs. Thus, substantial efforts have been made to simplify service and maintenance procedures so that the need for service calls by highly trained technicians and specialists can be minimized. Ideally, for example, all of the major work-stations of the printer, e.g. the charging, exposure, development, transfer and cleaning stations, as well as the recording element itself, should be replaceable by the end user or customer with no sacrifice being made to the ultimate image quality. Even where the services of a trained technician are required, the time to implement such replacements should be minimal. The achievement of this goal not only requires that each of the printer work-stations be modularized so as effect a "plug and play" concept, but also requires that a very precise and highly reliable work station-registration scheme be designed so that each work station, upon being removed from the printer frame for servicing and/or replacement, can be returned to within a few microns of its nominal position. In the case of high quality color printing, the respective placements of the printer work-stations is particularly critical and skilled servicing personnel are usually required to make the major sub-system changes. Obviously, the need for service assistance should be minimized.
In the above-referenced U.S. application Ser. No. 09/474,352, a work station registration scheme for an electrophotographic printer is disclosed in which a plurality of dowel pins on the printer frame serve to locate both a photoconductive drum assembly and an image transfer drum assembly. Each drum assembly comprises a pair of drum-support members, commonly referred to as "spiders," located at opposite ends of the drum. Each spider contains a centrally located bearing for rotatably supporting a drum axle, and a plurality of outwardly extending mechanical fiducials, e.g. bullet-shaped members, which are adapted to mate with complimentary structure, e.g., V-notched blocks, mounted on each work station to precisely locate and space the work-stations relative to the drum's photoconductive surface. When it comes time to replace the image-recording and/or transfer drums, the work-stations are retracted from their respective positions adjacent the drum surface, thereby providing clearance for drum removal, and the entire drum assembly, including the spider members, are slid axially through an opening in the front wall of the printer frame. The entire drum assembly is then returned to the manufacturer's facility where the assembly is disassembled and a new drum can be substituted for the worn drum. To install a new drum assembly, the reverse process is carried out, the drum assembly being moved axially inward into the printer frame, until the spiders engage and are seated upon the dowel pins. Thereafter, the work-stations are moved toward the drum surface and their respective operative positions. In a similar manner, each of the individual work-stations may be removed from the printer housing, leaving behind, when the drum assembly has been removed, a frame that is totally void of any major components. Only the registration dowel pins remain in the frame, and the entire printer can be reassembled with great precision based on the location of these pins.
While the above-described apparatus fulfills the work station-registration needs for high quality color printing, it may be viewed as a relatively costly and labor intensive solution. For example, to replace the drum surface, the entire drum assembly, which including the relatively costly drum-support members (spiders) and axles, must be replaced. This requires removal of a relatively heavy subsystem from the printer housing and shipment of such subsystem back to the manufacturing for refurbishing. Since most of the drum assembly components do not require replacement, these components undergo unnecessary shipping and handling during which time they may be damaged or have parts misaligned. This disadvantage is exacerbated by the weight of such components. Ideally, only those components of the drum assemblies that actually need replacement (typically the outermost layer of the drum) should be removed from the printer housing and the remaining components should stay in place for the life of the printer.